The present invention pertains to a process for reducing the corrosiveness to iron of an oxygenated aqueous solution by stripping with natural gases containing hydrogen sulfide acid in the presence of a deoxygenation catalyst. The invention also encompasses the composition used as the deoxygenation catalyst.
Water is reinjected into hydrocarbon deposits in order to maintain the deposit under a pressure which is sufficient for its exploitation and preservation. The injection waters are generally saline. In an off-shore environment, the waters are saline because seawater is employed to refill the deposits. On dry land, since fresh water is valuable for other applications, use is made of drilling water-.or recovery water, which are charged with mineral salts. These waters are also charged with dissolved oxygen, either because of their contact with the atmosphere or because of their applications which are never carried out in an inert atmosphere. It is known that oxygenated, saline waters are very corrosive to iron and steel. Therefore, the stripping technique is used to purge these waters of their dissolved oxygen. Deaerated gases are used for this purpose. In hydrocarbon fields, there is an abundant supply of gases essentially constituted of light hydrocarbons, but which contain H.sub.2 S gas in noteworthy proportions. Since deoxygenation by means of stripping is not total, the result is that the waters that could be treated with such gases and which still contain some oxygen, would be charged with hydrogen sulfide acid. Now, if we let V.sub., V.sub.2, V.sub.3 and V.sub.4 represent the corrosiveness of saline waters free of oxygen and hydrogen sulfide (V.sub.1), oxygenated only V.sub.2), only charged with hydrogen sulfide (V.sub.3), or charged with both oxygen and hydrogen sulfide (V.sub.4), respectively, then their corrosiveness can be ranked as follows: V.sub.4 &gt;&gt;V.sub.2 &gt;V.sub.3 &gt;&gt;V.sub.1, and thus the stripping of oxygen by means of petroleum gases is, at the very least, very unsatisfactory with regard to the conservation of metal installations.
Both the operation of prior desulfuration of the stripping gases, on the one hand, and the operation of total deoxygenation of the stripping water, on the other hand, are time-consuming and costly. In addition, they require bulky materials that cannot be shipped to off-shore platforms. The problem addressed by the applicant, and which has been resolved with the present invention, is the development of an additive that can limit the corrosion of installations by waters retaining oxygen in solution, particularly saline waters, in the presence of noteworthy amounts of hydrogen sulfide.
The problem has already been raised by the aggressiveness of H.sub.2 S brines, the corrosiveness of which is considerably increased when they are charged with oxygen from simple contact with the atmosphere. A solution to this problem was disclosed in U.S. Pat. No. 3,618,667 of E. Snavely (Mobil Oil Corporation), according to which the parasitic oxygen is eliminated by catalyzing its reduction by the H.sub.2 S gas present by addition of transition metal salts. Thus, the level of corrosion is reduced to that which characterizes non-oxygenated H.sub.2 S solution. Since then, the mechanisms of the catalytic reaction of oxygen on dissolved hydrogen sulfide acid have been scientifically analyzed by 0. Were, S.L. Tsao and R.M. Chhatre in Corrosion, Vol. 41, Issued 6, 1985.
It was found that this mechanism of catalytic reduction could be used to obtain the reduction of the corrosiveness of oxygenated saline waters, by stripping them with a stream of gas not containing oxygen but, in contrast, charged with hydrogen sulfide acid. In order for such an operation to yield a valid industrial result, it is necessary that this stripping not create a situation of corrosive synergy by charging with hydrogen sulfide acid the saline aqueous fluid which has had its dissolved oxygen incompletely removed. Catalysis with transition metal salts in accordance with Snavely, of course, makes it possible to avoid such a situation. But the saline solution has now been charged with hydrogen sulfide acid and thus corrosion from oxygen has been replaced by corrosion from hydrogen sulfide acid which, although present with a less dramatic appearance, still remains at an unacceptable level. The combination of transition metal salts and the conventional inhibitors of H.sub.2 S corrosion (fatty chain cationic derivatives, notably salts of fatty amines, fatty-chain quaternary ammonium salts) do not provide the desired solution because these compounds have the annoying property, in this context, of blocking the catalysis of H.sub.2 S decomposition by oxygen.